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Aryl diazonium chemistry generates a covalently attached thin film on various materials. This chemistry has diverse applications owing to the stability, ease of functionalization, and versatility of the film. However, the uncontrolled growth into a polyaryl film has limited the controllability of the film's beneficial properties. In this study, we developed a multistep grafting protocol to densify the film while maintaining a thickness on the order of nanometers. This simple protocol enabled the full passivation of a nitrophenyl polyaryl film, completely eliminating the electrochemical reactions at the surface. We then applied this protocol to the grafting of phenylphosphorylcholine films, with which the densification significantly enhanced the antifouling property of the film. Together with its potential to precisely control the density of functionalized surfaces, we believe this grafting procedure will have applications in the development of bioelectrical interfaces.

We investigate the response speed of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes by in situ electrical monitoring, which are grafted by surface-initiated activators regenerated b...

In this paper, we proposed to enhance a signal-to-noise (S/N) ratio for detecting a primary stress marker, serotonin, using a potentiometric biosensor modified by a well-designed nanofilter film. An extended-Au-gate field-effect transistor (EG-Au-gate FET) biosensor exhibits highly sensitive electrochemical detection toward various small biomolecules, including serotonin. Therefore, to enhance the S/N ratio for the serotonin detection, we designed an appropriate nanofilter film on the Au electrode by combining the aryldiazonium salt reduction strategy and boronate affinity. That is, only serotonin can approach the Au sensing surface to generate an electrical signal; interfering biomolecules are prevented from penetrating through the nanofilter, either because large interfering biomolecules cannot permeate through the highly dense, nanoporous multilayer film, or because phenylboronic acids included in the nanofilter captures small interfering biomolecules (e.g., catecholamines). The potentiometric biosensor modified by such a nanofilter film detected serotonin in a model sample solution containing catecholamines, cortisol, and human serum albumin with a high S/N ratio for the serotonin levels in the blood. Furthermore, we found that the effect of the nanofilter directly reflects the binding affinity of the receptors such as phenylboronic acids included in the nanofilter; thus, the selectivity and dynamic range of small target biomolecules can be tuned freely by designing the appropriate receptors for the nanofilter. The results show that a well-designed nanofilter biointerface can be a versatile biosensing platform for point-of-care testing, particularly for a simple stress check.

For enzyme-/antibody-free and label-free biosensing, a molecularly imprinted polymer (MIP)-based membrane with phenylboronic acid (PBA) molecules, which induces the change in the density of molecular charges based on the small biomolecule-PBA diol binding, has been demonstrated to be suitable for the bioelectrical interface of biologically coupled gate field-effect transistor (bio-FET) sensors. MIP-coated gate FET sensors selectively detect various small biomolecules such as glucose, dopamine, sialic acid, and oligosaccharides without using labeled materials. In particular, the well-controlled MIP film by surface-initiated atom transfer radical polymerization (SI-ATRP) contributes to the quantitative analysis of small biomolecule sensing, resulting in potentiometric Langmuir isotherm adsorption analysis by which the parameters such as the binding affinity between small biomolecules and MIP cavities are evaluated. Also, the output electrical signal of even a random MIP-coated gate FET sensor is quantitatively analyzed using the bi-Langmuir adsorption isotherm equation, showing the adsorption mechanism of small biomolecules onto the template-specific MIP membrane. Thus, a platform based on the MIP bioelectrical interface for the bio-FET sensor is suitable for an enzyme-/antibody-free and label-free biosensing system in the fields of clinical diagnostics, drug discovery, the food industry, and environmental research.

In this paper, we propose a novel concept of a biointerface, a polymeric nanofilter, for the potentiometric detection of small biomolecules using an extended-Au-gate field-effect transistor (EG-Au-FET). A Au electrode has the potential capability to detect various small biomolecules with ultrasensitivity at nM levels on the basis of a surface redox reaction, but it exhibits no selective response to such biomolecules. Therefore, a suitable polymeric nanofilter is designed and modified on the Au electrode, so that a small target biomolecule reaches the Au surface, resulting in an electrical signal, whereas low-molecular-weight interferences not approaching the Au surface are captured in the polymeric nanofilter. The polymeric nanofilter is composed of two layers. The first layer is electrografted as an anchor layer by a cyclic voltammetry method. Then, a filtering layer is precisely polymerized as the second layer by a photo-mediated surface-initiated atom transfer radical polymerization method. The thickness and density of the polymeric nanofilter are controlled to specifically detect a small target biomolecule with high sensitivity. As a model case, l-cysteine as the small target biomolecule at nM levels is specifically detected by filtering l-DOPA as a low-molecular-weight interference using the polymeric nanofilter-grafted EG-Au-FET on the basis of the following mechanism. The phenylboronic acid (PBA) that copolymerizes with the polymeric nanofilter captures l-DOPA through diol binding, whereas l-cysteine reaches the Au surface through the filter layer. The polymeric nanofilter can also effectively prevent the interaction between biomacromolecules such as albumin and the Au electrode. A platform based on a polymeric nanofilter-grafted EG-Au-FET biosensor is suitable for the ultrasensitive and specific detection of a small biomolecule in biological samples such as tears and sweat, which include small amounts of low-molecular-weight interferences, which generate nonspecific electrical signals.

In this paper, we demonstrate the use of a monolayer film electrografted via diazonium chemistry for controlling the potential response of a field-effect transistor (FET)-based sensor. 4-Nitrobenzenediazonium salt is electrografted on an extended-Au-gate FET (EG-Au-FET) with or without using a radical scavenger by cyclic voltammetry (CV), resulting in the formation of a monolayer or multilayer. In particular, the surface coverage of the aryl-derivative monolayer on the Au gate electrode gradually increases with increasing number of potential cycles in CV. Here, Au exhibits a strong catalytic action, resulting in the oxidation of organic compounds. Uric acid is used as a low-molecular-weight biomolecule for interference. The denser the surface coverage of the grafted monolayer, the smaller the potential response of the EG-Au-FET because the redox reaction of uric acid with the Au gate surface is suppressed. On the other hand, the effect of the aryl-derivative multilayer on the suppression of the potential response was smaller than that of the monolayer because the electrogenerated aryl radicals did not react with the Au surface but with the grafted species, resulting in an exposed part of the Au surface among the grafted aryl molecules. Thus, a platform based on such a monolayer film electrografted via diazonium chemistry is suitable for controlling the potential response based on the interference of low-molecular-weight biomolecules in biosamples.

In this paper, we report the concept of an aptamer (Ap)-based nanofilter interface for the suppression of nonspecific signals generated by interfering species in biological samples. As a model of the target small biomolecule, l -3,4-dihydroxyphenylalanine ( l -DOPA) is used to demonstrate its electrical discrimination from dopamine (DA) despite their similar chemical structures using a DA–Ap nanofilter-coated Au gate field-effect transistor (FET). A Au gate electrode is employed to assess the electrical response of the FET to l -DOPA on the basis of oxidative reactions. In particular, the proposed nanofilter interface is based on the DA–Ap layer immobilized at the aryl-diazonium-based anchor monolayer, which is electrochemically grafted on the Au gate electrode via diazonium chemistry. As a result, the electrical signal from l -DOPA is clearly discriminated from that from DA at a concentration of 1 μM or higher using the DA–Ap nanofilter-coated Au gate FET. That is, DA is trapped by the DA–Ap nanofilter through the formation of complexes, the molecular charges of which are shielded by counter ions that are not in contact with the Au gate surface, whereas l -DOPA reaches and electrochemically reacts with the Au gate surface, generating an electrical signal regardless of the nanofilter. The surface properties of the DA–Ap nanofilter modified on the Au gate electrode are investigated by cyclic voltammetry, chronocoulometry, atomic force microscopy, and X-ray photoelectron spectroscopy. A platform based on the FET biosensor with the Ap-based nanofilter interface contributes to the electrical monitoring of small biomarkers while suppressing the nonspecific signals of interfering species in biological samples.

Small biomolecules contained in body fluids are often recognized as indicators of diseases and health conditions. Therefore, the development of biosensors capable of recognizing significant biomarkers from body fluids such as blood, sweat, tears, and saliva, leads to early diagnosis and contribute to the realization of next-generation medical care. In this regard, we have recently developed the potentiometric biosensor based on the extended-gate field-effect transistor (EG-FET) for small-biomolecule recognition. The catalytically active extended-Au electrodes have the capability to detect various small biomolecules with high sensitivity on the basis of the surface electrochemical reaction. However, since the Au electrode may react with various electrochemically active biomolecules, it is necessary to limit the interfering biomolecules from approaching the Au surface in order to enhance target selectivity. Therefore, a suitable polymeric nanofilter was developed on the Au electrode where interfering biomolecules are prevented from penetrating through the nanofilter, and only the target biomolecules can access the Au sensing surface to generate an electrical signal.1,2 The polymeric nanofilter biointerface is composed of two layers. The first layer is an anchor layer directly grafted on the Au surface, which controls the density of the polymeric nanofilter to make enough space for only small biomolecules to penetrate through and prevent large-biomolecule interferences from approaching the Au sensing surface. Then, a polymeric filter layer as the second layer is precisely grafted from the surface of the anchor layer by atom transfer radical polymerization (ATRP). The filter layer selectively captures the small-biomolecule interferences outside the Debye’s length; therefore, the interfering small-biomolecules will not cause the change in Au surface potential. In our previous study, we designed and developed the polymeric nanofilter by using aryldiazonium reduction chemistry and boronate affinity. Aryldiazonium reduction chemistry enabled the grafting of chemically stable multilayer film on the Au surface, having an adequate density in order to suppress the penetration of large biomolecules. Phenylboronic acid was included in the polymeric filter layer to capture the model interfering small biomolecule, L-DOPA, via a reversible PBA/diol boronate affinity binding. Resultantly, the polymeric nanofilter-coated EG-FET biosensor specifically detected the model target biomolecule, L-cysteine. In this study, we designed and developed the second polymeric layer on the polymeric nanofilter to give an additional function. Although the space-controlled nanofilter suppresses the penetration of the large biomolecules, the non-specific adsorption can cause severe fouling of the nanofilter surface. Therefore, in order to improve the anti-fouling function of the nanofilter interface, poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC) was chosen as the second polymeric layer. PMPC is a bio-inspired polymer that has been widely used as a biocompatible, anti-fouling coating material. In order to investigate the anti-fouling function of pMPC coating, we firstly grafted the pMPC layer directly from the anchor layer surface via surface-initiated ATRP. Then, the response of pMPC-nanofilter coated EG-FET biosensor to human-serum albumin (HSA) was compared with the response of the control (unmodified) sensor. Resultantly, the pMPC-coated sensor suppressed the non-specific response up to 60% compared with the control sensor. Therefore, we verified the successful enhancement of the anti-fouling property on the nanofilter surface. Reference (1) Nishitani, S.; Sakata, T. ACS Appl. Mater. Interfaces 2019, 11 (5), 5561–5569. (2) Himori, S.; Nishitani, S.; Sakata, T. Langmuir 2019, 35 (10), 3701–3709.

The origin of the unusually high stability of the sialic acid (SA) and phenylboronic acid (PBA) complex was investigated by a combined nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) study. SA is a glycan‐terminating monosaccharide, and its importance as a clinical target has long been recognized. Inspired by the fact that the binding properties of SA–PBA complexation are anomalously high relative to those of typical monosaccharides, great effort has been made to build a clinical platform with the use of PBA as a SA‐selective receptor. Although a number of applications have been reported in recent years, the ability of PBA to recognize SA‐terminating surface glycans selectively is still unclear, because high‐affinity SA–PBA complexation might not occur in a physiological environment. In particular, different forms of SA (α‐ and β‐pyranose) were not considered in detail. To answer this question, the combined NMR spectroscopy/DFT study revealed that the advantageous binding properties of the SA–PBA complex arise from ester bonding involving the α‐carboxylate moieties (C1 and C2) of β‐SA but not α‐SA. Moreover, the facts that the C2 atom is blocked by a glycoside bond in a physiological environment and that α‐SA basically exists on membrane‐bound glycans in a physiological environment lead to the conclusion that PBA cannot selectively recognize the SA unit to discriminate specific types of cells. Our results have a significant impact on the field of SA‐based cell recognition.

An artificially well-designed biointerface is required for a new strategy of biosensor such as an enzyme-free glucose sensor. A solution-gate field-effect transistor (FET) is based on the potentiometric measurement of intrinsic ionic or biomolecular charges at the gate surface of the FET without enzymatic reactions. A platform based on this measurement method is suitable for the detection of biomarkers in a noninvasive, real-time, and label-free manner. Moreover, an extended-Au-gate FET enables a highly-sensitive detection of small biomarkers, because the Au surface exhibits a strong catalytic action, resulting in the oxidation of organic compounds such as uric acid and glucose. However, the specific detection of small biomarkers, which involves a signal to noise ratio, with the Au electrode is not sufficient for a real sample containing impurities. To prevent the nonspecific signals based on impurities, the surface of Au electrode should be chemically covered with a functional membrane. Therefore, we propose to chemically graft the aryl-derivative film on the Au electrode in this study, because the surface coverage of aryl-derivative film can be electrochemically controlled in a simple way. In particular, we design a Au film/solution interface for the specific detection of small biomarkers and investigate the effect of aryl-derivative monolayer on the prevention of nonspecific signals based on non-targeted small biomarkers. A Au(/Cr) film was fabricated by sputtering on a glass substrate. The Au electrode was modified by nitro group-terminated aryldiazonium monolayer in acetonitrile including 1 mM 4-nitrobenzenediazonium tetrafluoroborate (NBD), 25 mM tetrabutylammonium hexafluorophosphate (nBuPF6), and 2 mM DPPH via cyclic voltammetry (CV). DPPH was used as radical scavenger to make aryl-derivative monolayer. The immobilization density of aryl-derivative monolayer was controlled via the CV cycles. On the other hand, the same method was applied for the modification of aryl-derivative multilayer in the solvent without DPPH. The change in the surface potential was monitored using the aryl-derivative film-grafted FET. In particular, uric acid was used as a small interference molecule and introduced onto the Au electrode with the aryl-derivative monolayer or aryl-derivative multilayer, the density of which was varied. The concentration of added uric acid was changed in the range of 5 µM to 1 mM. The thickness of aryl-derivative film was analyzed using laser microscope. Besides, the surface coverage was calculated by the reduction from nitro groups of NBD to amino groups. In addition, the surface properties of aryl-derivative films were evaluated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The uric acid response for the aryl-derivative monolayer-grafted FET was smaller than that for the aryl derivative multilayer-grafted FET. That is, the aryl-derivative monolayer-grafted FET suppressed the nonspecific electrical signal based on the interaction of uric acid with the Au surface. This is because the surface coverage by the aryl-derivative multilayer was insufficient, that is, there were the exposed part of the Au surface among the grafted aryl molecules, through which uric acid approached to the Au surface, while the density of the aryl-derivative monolayer grafted on the Au surface was higher than that of the aryl-derivative multilayer. Moreover, the effect of the aryl-derivative monolayer on the prevention of nonspecific signal was enhanced with increasing the CV cycles for electrografting, resulting in the increase of immobilization density. Thus, a platform based on such a monolayer film electrografted via diazonium chemistry is suitable for controlling the potential response based on the interference of small molecules in biosamples. Our work suggests a new strategy for the specific detection of small biomarkers using the extended-Au-gate FET biosensor. Now, we design a polymeric nanofilter interface on the Au electrode, which is polymerized from the aryl-derivative films, to utilize the Au electrode with the high sensitivity. In this scheme, the polymeric nanofilter has a functional group to capture small interference molecules outside the diffusion layer near the Au electrode. Biomacromolecules such as proteins can’t access to the Au surface, which means the prevention of nonspecific adsorptions, owing to the high density of grafted films. In addition, small interference molecules can’t also access to the Au surface because the functional groups in the polymeric nanofilter catches them. Then, a target small biomarker can only access to the Au surface and be detected owing to a catalytic action. For this scheme, it is very important to control the density of aryl-derivative films. Figure 1

Semiconductor-based biosensors are suitable for the detection of small molecules such as saccharides as long as they have molecular charges. In this study, we developed molecularly imprinted polymer-based field effect transistor (MIP-gate FET) for selective sugar chain sensing in aqueous media. Choosing 3'-sialyl lactose (3'-SLac) and 6'-sialyl lactose (6'-SLac) as target sugars, we confirmed that they could be sensed quantitatively from 10uM. Moreover, the sensor showed selectivity to some extent, where the signal from competent was suppressed by 40% at maximum. We also found that the selectivity enhanced by MIP differs for 2 different template sugars, although these sugars are similar in structures. Moreover, the selective detection of other oligosaccharides was investigated using the MIP-gate FET. From these results, we showed the possibility of MIP-gate FET as selective sugar detecting sensor, which can be applied widely in medical fields.

In this study, we developed a molecularly imprinted polymer-based field-effect transistor (MIP-gate FET) for selectively detecting sugar chains in aqueous media, focusing on 3'-sialyllactose (3SLac) and 6'-sialyllactose (6SLac). The FET biosensor enables the detection of small molecules as long as they have intrinsic charges. Additionally, the MIP gels include the template for the target molecule, which is selectively trapped without requiring enzyme-target molecule reaction. The MIP gels were synthesized on the gate surface of the FET device, including phenylboronic acid (PBA), which enables binding to sugar chains. Firstly, the 3SLac-MIP-gate FET quantitatively detected 3SLac at µM levels. This is because the FET device recognized the change in molecular charges on the basis of PBA-3SLac binding in the MIP gel. Moreover, 3SLac was selectively detected using the 3SLac- and 6SLac-MIP-gate FETs to some extent, where the detecting signal from the competent was suppressed by 40% at maximum. Therefore, a platform based on the MIP-coupled FET biosensor is suitable for a selective biosensing system in an enzyme-free manner, which can be applied widely in medical fields. However, we need to further improve the selectivity of MIP-gate FETs to discriminate more clearly between similar structures of sugar chains such as 3SLac and 6SLac.